In this proposal, I describe a series of experiments that will investigate the role of PSD-95 in synaptic transmission, synaptic plasticity and drug-induced synaptic plasticity in dopamine neurons in the ventral tegmental area (VTA). This will involve making whole-cell patch clamp recordings from VTA dopamine neurons in a slice preparation prepared from PSD-95 knockout or wildtype mice. I will also perform molecular "rescue" experiments that will entail using viral-mediated gene transfer to express recombinant PSD-95 fused to GFP in VTA dopamine cells from PSD-95 knockout mice. Multiple forms of long-term potentiation (LTP) and long-term depression (LTD) have been described in VTA dopamine neurons and it is known that in vivo administration of drugs of abuse can elicit synaptic plasticity in these cells. Yet little is known about the molecular mechanisms underlying these phenomena. An extensive body of work primarily on hippocampal synapses suggests that the postsynaptic scaffolding protein, PSD-95, plays an important role in excitatory synaptic function and plasticity. Additionally, PSD-95 knockout mice have recently been shown to have alterations in drug-induced behavioral plasticity. The role of PSD-95 in synaptic and drug induced plasticity in VTA dopamine neurons, however, has not been examined. I will first investigate whether basal synaptic transmission is altered in VTA dopamine neurons from PSD-95 knockout mice. I will then test whether PSD-95 knockout mice exhibit normal LTP and LTD in these cells. Next I will determine if PSD-95 knockouts have altered drug-induced plasticity in VTA dopamine neurons in response to in vivo administration of previously characterized drugs of abuse. Finally, I will test whether observed alterations in synaptic function and plasticity in the PSD-95 knockout can be rescued by expression of recombinant PSD- 95 in VTA dopamine neurons. This will be accomplished using a lentivirus expressing PSD-95 fused to GFP that will be stereotaxically injected into the VTA. The results of these experiments should significantly enhance our knowledge of the molecular mechanisms underlying synaptic plasticity in VTA dopamine neurons as well as the role of PSD-95 in drug-induced plasticity. By furthering our understanding of how the reward circuitry of the brain works we can better understand the brain mechanisms that lead to addiction and thereby also develop better treatments for this devastating illness. The research proposed in this grant application is aimed at elucidating some of the drug-induced molecular changes that occur in the reward circuitry, which underlie the lasting behavioral modifications that define addiction.